The invention relates to a drive apparatus for an injection molding machine.
Conventionally, in an injection molding machine, resin heated and melted in a heating cylinder is injected into a cavity of a mold apparatus under high pressure so that the cavity is filled with the molten resin. The molten resin is then cooled and solidified so as to obtain a molded article.
The mold apparatus comprises a stationary mold and a movable mold. The movable mold is advanced and retracted by a mold clamping apparatus so that the movable mold is attached to and separated from the stationary mold, to thereby perform mold closing, mold clamping and mold opening.
The mold clamping apparatus has a toggle mechanism to advance and retract the movable mold. The toggle mechanism is driven by an electric motor or a servomotor in a drive section.
FIG. 1 is a sectional view of a drive section of a conventional mold clamping apparatus.
In FIG. 1, the servomotor 31 as a drive means has a motor case which comprises a plate-shaped first flange 54 to attach the servomotor 31 to a toggle support not illustrated, a plate-shaped second flange 55 spaced apart from the first flange 54, and a cylindrical frame 62 disposed between the first flange 54 and the second flange 55. Inside the motor case, a rotor 60 and a stator 61 are disposed.
A hollow output shaft 50 is rotatably disposed relative to the motor case. Its rear end (leftward end in FIG. 1) is supported by a thrust bearing 57 while its front end (rightward end in FIG. 1) is supported by a thrust bearing 58. By these thrust bearings 57 and 58, the output shaft 50 is supported in a thrust direction and is rotatably supported in a radial direction. To rotate the output shaft 50, the stator 61 is fixed to the frame 62 and the rotor 60 is fixed to the output shaft 50. A coil 45 is mounted to the stator 61. Bolts 59 connect the first flange 54 and the second flange 55. By tightening the bolts 59, the frame 62 is pressed against the first flange 54 by the second flange 55. An encoder 48 is attached to the second flange 55 via a bracket 47.
At the rear end (leftward end in FIG. 1) of the output shaft 50, a fixing nut 46 is threadably engaged with it. At the front end (rightward end in FIG. 1) of the output shaft 50, a nut 51 is fixed to it by bolts 53. Therefore, by tightening the bolts 53, the thrust bearings 57 and 58 are pressed by the fixing nut 46 and the nut 51.
A screw shaft 63 extends inside the output shaft 50, engages threadably with the nut 51, and then further extends forward (rightward in FIG. 1) to a cross head not illustrated. Its front end is connected to the cross head on which a toggle mechanism is disposed. The cross head is prevented from rotating by a guide bar not illustrated. A drive section comprises the servomotor 31, the output shaft 50, the nut 51, the screw shaft 63 and the cross head.
Consequently, when a current of electricity is supplied to the coil 45 and the servomotor 31 is driven, the rotor 60 is rotated. The rotation is sequentially transmitted to the output shaft 50 and the nut 51. By the engagement of the nut 51 and the screw shaft 63, the rotation (in the direction of A in FIG. 1) of the nut 51 is transformed to rectilinear motion (in the direction of B in FIG. 1) of the screw shaft 63. Thus, the screw shaft 63 and the cross head are advanced and retracted (moved rightward and leftward in FIG. 1) in a stroke Sb.
When the screw shaft 63 is advanced (moved rightward in FIG. 1), the cross head is also advanced so that the toggle mechanism extends and advances the movable platen not illustrated, to thereby perform mold closing and mold clamping. When the screw shaft 63 is retracted (moved leftward in FIG. 1), the cross head is also retracted so that the toggle mechanism contracts the movable platen, to thereby perform mold opening.
However, in the above conventional drive section, the output shaft 50 and the nut 51 are rotated to advance and retract the screw shaft 63 so that its inertia is extremely large. Therefore, when the servomotor 31 is driven, the response of mold opening/closing is delayed so that molding cycle becomes long. Further, vibration is caused in the drive section when the servomotor 31 is started and stopped.
Accordingly, a mold clamping apparatus with a fixed nut and a rotated screw shaft is provided.
FIG. 2 is a sectional view of a drive section of another conventional mold apparatus. With respect to the elements which have the same structure as the mold clamping apparatus in FIG. 1, the explanations of them are omitted by giving the same numeral to them.
In this apparatus, the hollow output shaft 50 is rotatably disposed relative to a motor case 64. A spline nut 68 is disposed about the center of an inner cylindrical surface of the output shaft 50. The nut 51 is fixed to a front end (rightward end in FIG. 2) of the motor case 64.
A screw shaft unit 65 extends inside the output shaft 50 and the nut 51. A cross head not illustrated is attached to the screw shaft unit 65 at its front end (rightward end in FIG. 2) via bearings not illustrated. In the screw shaft unit 65, a spline portion 66 is formed in its rear (leftward in FIG. 2) while a screw shaft portion 67 is formed in its front (rightward in FIG. 2). The spline portion 66 engages slidably and matably with the spline nut 68. The screw shaft portion 67 engages threadably with the nut 51. A drive section comprises the servomotor 31, the output shaft 50, the nut 51, the screw shaft unit 65 and the cross head.
Consequently, when the servomotor 31 is driven and the rotor 60 is rotated, the rotation is sequentially transmitted to the output shaft 50, the spline nut 68 and the screw shaft unit 65. By engagement of the nut 51 and the screw shaft portion 67, the rotation of the screw shaft unit 65 is transformed to rectilinear motion. Thus, the screw shaft unit 65 and the cross head are advanced and retracted (moved rightward and leftward in FIG. 2).
In this conventional apparatus, the inertia of the drive section becomes small because the nut 51 is not rotated to advance and retract the screw shaft unit 65. Therefore, when the servomotor 31 is driven, the response of mold opening/closing becomes rapid so that molding cycle becomes short. Further, the vibration in the drive section is prevented when the servomotor 31 is started and stopped.
However, the spline portion 66 and the screw shaft portion 67 are disposed in series along the screw shaft unit 65 in an axial direction to transmit the rotation of the output shaft 50 to the screw shaft portion 67. Therefore, the spline portion 66 makes the size of the drive section large.